Disconnect the cable from the negative battery terminal, then drain the cooling system. Remove the engine cover and the air intake duct assembly, followed by disconnecting the accelerator cable. Disconnect the electrical connectors from the Throttle Position sensor, Idle Speed Actuator, Variable Intake Solenoid actuator, fuel injectors, Purge Control Solenoid Valve, and Intake Air Temperature sensor, labeling each connector for correct reassembly and pulling the wiring harnesses aside. Disconnect the EVAP hose from the Purge Control Solenoid Valve, the brake booster vacuum hose, and the Positive Crankcase Ventilation valve hose. Remove the ground strap bolt and disconnect the ground strap from the upper intake manifold, then disconnect the coolant hoses from the throttle body, clamping or plugging them to prevent spillage. Unbolt and remove the upper intake manifold support bracket, then remove the bolts mounting the upper intake manifold to the lower intake manifold, pulling it forward and detaching the accelerator and cruise control cables. If replacing the upper intake manifold, transfer all components to the new manifold, then install it on the lower manifold and tighten the fasteners in the correct sequence. Relieve the fuel system pressure, remove the upper intake manifold, and then remove the fuel rail and injectors. Detach the lower intake manifold from the engine by removing the manifold bolts in reverse order of the tightening sequence, ensuring not to pry between the gasket mating surfaces. Check the lower manifold mating surface for warping and resurface if necessary, then scrape off old gasket material and clean the mating surfaces. Install new gaskets, position the lower intake manifold on the engine, ensuring the gaskets haven't shifted, and tighten the bolts in three or four equal steps to the specified torque in the correct sequence. Install a new gasket between the lower and upper intake manifolds, place the upper intake manifold on the lower intake manifold, and tighten the bolts in the proper sequence to the specified torque. Reinstall or reconnect everything removed or disconnected from the upper intake manifold, check the coolant level, reconnect the battery, and run the engine to check for fuel, vacuum, and coolant leaks.